Dehydrating treatment of oil and gas wells

ABSTRACT

Water is removed from passages in a subterranean hydrocarbon formation by feeding into the passages a composition comprising 5 to 70 wt. % of alkanol(s) having 1 to 3 carbon atoms and 30 to 95 wt. % dense phase carbon dioxide, under a pressure greater than zero up to 25,000 psig and at a temperature of 0° F. to 200° F., allowing the feed composition to remain in the well, whereby the feed composition solubilizes with said water in said passages, and then removing from said well a liquid product composition comprising said solubilized water and alkanol.

This application claims priority from U.S. Provisional Application Ser.No. 60/872,724, filed Dec. 4, 2006, the entire content of which ishereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to treating oil and gas production wellsto improve their operational capacity.

BACKGROUND OF THE INVENTION

Production rates from wells in formations that contain oil or gas can beincreased by hydrofracturing, in which a fluid is applied at highpressure within the formation to induce fractures and fissures in theformation and thereby stimulate release of the desired hydrocarbons fromthe formation. The fluid typically contains a liquid vehicle andadditional solid and/or liquid components that enhance the penetrationof the hydrofracturing fluid into the areas of the formation that are tobe hydrofractured, and enhance the effectiveness of the fracturing thatis carried out.

However, if excessive amounts of water have accumulated in theformation, especially in the passages of the formation into which thehydrofracturing fluid would be applied, then the desired hydrofracturingcannot be carried out as fully because the water blocks thehydrofracturing fluid from reaching as far into the passages of theformation as would be possible if the blocking water were not present.

Thus, there remains a need for an effective method of removing inducedand conic water from the areas of an oil or gas formation so as toimprove the ability to stimulate production of hydrocarbons from theformation by hydrofracturing.

BRIEF SUMMARY OF THE INVENTION

The present invention solves these and other problems, and providesnumerous advantages described herein, in a method for dehydrating asubterranean oil-containing or gas-containing formation that containswater in passages in the formation, comprising

(a) feeding into a well that extends to said passages in said formationa feed composition comprising 5 to 70 wt. % of an alkanol componentselected from the group consisting of alkanols containing 1 to 3 carbonatoms and mixtures thereof, and 30 to 95 wt. % dense phase carbondioxide, under a pressure from zero up to 25,000 psig and at atemperature of 0° F. to 200° F.,

(b) allowing the feed composition to remain in the well, whereby thefeed composition solubilizes with said water in said passages, and then

(c) removing from said well a liquid product composition comprising saidsolubilized water and alkanol.

As used herein, “dense phase carbon dioxide” means carbon dioxide in theform it takes at a temperature 10° F. below its critical temperature orhigher and at a pressure 100 psi below its critical pressure or higher.

As used herein, a substance “solubilizes” or becomes “solubilized” whenit forms a pumpable liquid with another one or more materials, whetherby forming a solution or dispersion of the substance in the one or moreother materials, or a solution or dispersion of the one or more othermaterials in the substance.

DETAILED DESCRIPTION OF THE INVENTION

The composition used in the present invention contains, at least, densephase carbon dioxide and an alkanol component. The term “downholecomposition” will be used in the following description to refer to thecomposition when it is ready to be fed down an oil or gas well, meaningthat it contains all components that it will contain as it is fed downthe well and that it is at the desired temperature and pressure at whichit will be fed down the well.

The preferred dense phase carbon dioxide is supercritical carbondioxide, wherein the carbon dioxide is at a temperature above itscritical temperature of 87.8° F. and above its critical pressure of 1070psia. However, carbon dioxide that is at a temperature somewhat belowits critical temperature and/or at a pressure somewhat below itscritical pressure can be used effectively as well.

The dense phase carbon dioxide comprises at least 30 wt. % of thedownhole composition, and more preferably at least 50 wt. % of thedownhole composition. Higher amounts, up to 90 wt. % or even up to 95wt. % of the downhole composition, can be used with good results.

The alkanol component comprises at least 5 wt. % of the downholecomposition. The alkanol component can comprise 10 wt. % or more of thedownhole composition, up to 50 wt. % or even up to 70 wt. %. Generally,higher alkanol concentrations are preferred with higher amounts of watercreating pore throat blockages in the capillaries in the formationpassages to be dehydrated by the method of the present invention. Thepreferred alkanol component is methanol.

In one preferred embodiment of the present invention, the dense phasecarbon dioxide and the alkanol component are the only substances presentin the downhole composition. However, other compounds that are effectiveas solubilizers for the water in the formation can be present. It ispreferred however that aromatic compounds, that is, compounds containingone or more than one benzene ring, should not be present in the downholecomposition. Examples of such aromatic compounds include benzene,toluene, and xylenes.

One preferred manner for preparing for and carrying out the presentinvention includes the following steps. The alkanol component and thecarbon dioxide are combined in a vessel that can accommodate thepressures that are applied. This can be accomplished by, first,partially filling a suitable vessel with an amount of the alkanolcomponent necessary to obtain the desired alkanol/carbon dioxide ratio.Then, carbon dioxide is fed into the vessel in an amount necessary toprovide the desired ratio of the alkanol component to the carbondioxide. The carbon dioxide can be provided from a conventional cylinderor larger storage/supply container which typically holds carbon dioxideunder pressure. Any other components such as surfactant, additionalsolvent, and the like, are added to the vessel, or injected into thestream of carbon dioxide and alkanol while going downhole, in thedesired amounts thereof.

Next, the mixture of alkanol and carbon dioxide (and any othercomponents present) is heated, for instance by pumping it through a heatexchanger, to achieve the desired temperature. Satisfactory temperaturesare generally in the range of 70° F. to 170° F.

The mixture is then injected downhole. Preferably a nearly constant pumppressure is maintained, varying flow as necessary. Pressure up to 10,000psig and even up to 25,000 psig is applied. The mixture is helddownhole, by not releasing the pressure on the well opening, for aperiod of time sufficient to allow the mixture to solubilize water inthe formation passages. Suitable hold times range from several minutesup to 24 hours. One way to determine an effective length of time is todetermine the pressure at the well opening when injection of the feedmixture is completed, and to permit the mixture to remain in the welluntil the pressure has decreased by a predetermined amount such as by25%. The decrease in pressure indicates that the feed mixture haspenetrated into the formation and solubilized water as desired.

After the desired hold time, the pressure on the mixture is released,allowing the mixture of solubilized water and alkanol to flow out of thewell. Water removed from the formation passages can be replaced byhydrofracturing fluid fed into the well.

Preparing the downhole composition for injection into the well requiresa high pressure pump having a minimum 5000 psig working pressure; and aheater capable of heating the composition to the preferred supercriticaltemperature. One suitable heater is a direct-fired heater. Anotheruseful heater uses an indirect line heater using glycol as the heatingmedium and tubes rated to the maximum discharge of the pump at 300° F.Preferably the temperature of the downhole composition is set at a valuewithin 10° F. of the temperature at the bottom of the well, if possibleor at least mid hole temperature, to minimize the risk of imposingthermal shock on the piping at the bottom of the well.

Injection of the carbon dioxide into the wellbore to control itspressure can be carried out in any of several ways, preferablyaccompanied by controlling its critical pressure for maximum heatingresults. The preferred way which in some cases, which is also moreeconomical, is through use of tubing which is brought to the oil or gaswell location coiled on a portable unit, rigged up next to the well, anduncoiled and fed into the wellbore, preferably to the full formationdepth or short thereof. The heated composition is then injected into thecoil tubing. The interior dimensions of the coil tubing would be sizedfor to allow the carbon dioxide to remain above its critical pressureallowing best heating control and maximum heat. A second way, whoseadaptation would depend principally on economics, would be to set apacker using the wells production tubing and then injecting the mixturedown the tubulars.

After the composition has been held in the well for the desired periodof time, the pressure being applied to the well at its opening isdecreased or removed, whereupon a flowable, pumpable liquid compositionis removed from the well. This removal can occur without external aid,relying on the internal pressures within the well itself or on remainingenergy from the injected carbon dioxide, or can be aided by a suitablepump that withdraws the liquid from the well. If the well is allowed toflow back by itself the flow back should be controlled using a chokevalve, increasing the flow back incrementally after the pressure beginsto drop.

The composition that is recovered from the oil or gas well is a liquidthat comprises water and alkanol that had been a component of thedownhole composition.

The method of the present invention has been shown to be effective inremoving undesired water from within the well, resulting in improvedproduction rates and economic value for the treated well when the wellis subsequently subjected to a given hydrofracturing treatment.

A preferred aspect of the present invention is the ability to use polarand non-polar solvents, alcohol and carbon dioxide, in wells andformations that do not contain accumulations of hydrocarbon solids (bywhich is meant hydrocarbons, namely compounds composed predominantly ofcarbon and hydrogen, that are solid at the conditions existing in an oilor gas well or at the conditions existing at the earth's surface outsidethe well, and which typically comprise paraffins and asphaltenes). Theabsence of such solids means that alkanol and carbon dioxide fed intothe formation are not consumed in solubilizing solid hydrocarbons, sothat more of the mixture of alkanol and carbon dioxide is available forsolubilization and removal of water from the formation.

Another advantage is that the carbon dioxide is believed to establish anacidic pH in the composition that is fed downhole, and in the formationpassages into which the composition passes, that reduces the tendency ofclays present in the passages to swell. This reduced swelling would alsoenhance the ability of the hydrofracturing fluid to reach further intothe formation passages where hydrofracturing is to occur.

In addition, oil produced after treatment of an oil well by the methodof the present invention has contained acceptable parts per million ofresidual alcohol levels.

Another advantage is that the method of the present invention is carriedout at the same well from which production of oil or gas will resume orwill be enhanced, as distinguished from other technologies whereincompositions are injected into one or more injection wells some distancefrom the production well itself in the hope of increasing the flow ofhydrocarbons out of the production well without putting anything downthe production well.

1. A method for dehydrating a subterranean oil-containing orgas-containing formation that contains water in passages in theformation, comprising (a) feeding into a well that extends to saidpassages in said formation a feed composition comprising 5 to 70 wt. %of an alkanol component selected from the group consisting of alkanolscontaining 1 to 3 carbon atoms and mixtures thereof, and 30 to 95 wt. %dense phase carbon dioxide, under a pressure from zero up to 25,000 psigand at a temperature of 0° F. to 200° F., (b) allowing the feedcomposition to remain in the well, whereby the feed compositionsolubilizes with said water in said passages, and then (c) removing fromsaid well a liquid product composition comprising said solubilized waterand alkanol.
 2. A method according to claim 1 wherein said feedcomposition comprises 50 wt. % to 95 wt. % dense phase carbon dioxide.3. A method according to claim 1 wherein said feed composition comprises5 wt. % to 50 wt. % of said alkanol component.
 4. A method according toclaim 1 wherein said feed composition is fed at a temperature of 70° F.to 170° F.
 5. A method according to claim 1 wherein said feedcomposition is free of aromatic compounds.
 6. A method according toclaim 1 wherein said formation into which said feed composition is feddoes not contain an accumulation of hydrocarbon solids.
 7. A methodaccording to claim 1 wherein said alkanol component comprises methanol.8. A method according to claim 7 wherein said feed composition comprises50 wt. % to 95 wt. % dense phase carbon dioxide.
 9. A method accordingto claim 7 wherein said feed composition comprises 5 wt. % to 50 wt. %of methanol.
 10. A method according to claim 7 wherein said feedcomposition is fed at a temperature of 70° F. to 170° F.
 11. A methodaccording to claim 7 wherein said feed composition is free of aromaticcompounds.
 12. A method according to claim 7 wherein said formation intowhich said feed composition is fed does not contain an accumulation ofhydrocarbon solids.